Technical Field
The present invention relates to a radiation image capture device, a control method for an erasing light source, and a computer-readable storage medium.
Related Art
Known radiation image capture devices, such as mammography devices, are provided with a radiation detector (Flat Panel Detector: FPD) to perform X-ray imaging of a breast. Radiation detectors are classified into direct conversion method types and indirect conversion method types according to their different conversion methods. Direct conversion method type radiation detectors convert X-ray data directly into electrical signals in a photoconductor layer employing amorphous selenium (a-Se).
Direct conversion method type radiation detectors are capable of achieving both high sharpness and high sensitivity. However, it has been observed that radiation detectors employing amorphous selenium (a-Se) in the photoconductor layer are prone to lag images due to slow transportation of charges arising in the photoconductor layer. In order to erase such lag images, light (referred to below as erasing light) is illuminated toward the radiation detector to generate photoelectric charges inside the photoconductor layer, promoting the disappearance of residual charge remaining in the photoconductor layer.
For example, Japanese Patent Application Laid-Open (JP-A) No. 2009-11526 (Patent Publication 1) includes an imaging condition setting means that sets imaging conditions of X-rays irradiated from an X-ray tube, a light illuminating section that illuminates a backlight for erasing lag images in an X-ray detector, a voltage value setting means that sets a voltage value for the backlight illuminated by the light illuminating section, and an imaging condition-voltage value correspondence table that holds most appropriate backlight voltage values for erasing lag images, derived according to the imaging conditions set by the imaging condition setting means. The most appropriate backlight voltage value is extracted from the imaging condition-voltage value correspondence table according to the imaging conditions, the voltage value setting means changes to this backlight voltage value, and the backlight is illuminated.
Moreover, JP-A No. 2002-296713 (Patent Publication 2) describes a control method for causing photostimulated luminescence to be emitted by illuminating excitation light onto a photostimulable phosphor plate that has been formed with a latent image by radiation irradiated from a radiation source, and, after reading image data based on the photostimulated luminescence, erasing an lag image in a radiation image capture device by illuminating the photostimulable phosphor plate with erasing light after releasing. In cases in which imaging is performed plural times using the same photostimulable phosphor plate, an amount of radiation with which the photostimulable phosphor plate will be irradiated at the next imaging is predicted, and an illumination duration of the erasing light is varied.
Known mammography devices are installed with a tomosynthesis imaging function, in which plural projection images acquired by irradiating radiation from plural directions are reconstituted to generate tomosynthesis images. In tomosynthesis imaging, plural projection images are successively captured while sequentially switching the radiation irradiation direction.
In tomosynthesis imaging, it is not desirable to erase lag images by repeatedly switching an erasing light on and off in coordination with the radiation irradiation timings over plural shots, since offset variation occurs in image signals, causing variation in the density values of radiation images. Photoelectric charges arise in a photoconductive layer due to activating the erasing light, and although the photoelectric charges disappear naturally with deactivation of the erasing light, there is a delay in the generation and disappearance of the photoelectric charges with respect to the activation and deactivation timings of the erasing light. Accordingly, if the erasing light is repeatedly activated and deactivated at short cycles, charges inside the photoconductive layer does not attain a stable state, causing offset variation in image signals.
There is also an issue of non-uniformities arising in radiation images if radiation image capture is performed in an activated state of the erasing light. The photoelectric charge arising inside the photoconductive layer due to illumination with the erasing light is not uniform, due to such factors as unevenness in the layer thickness of the respective layers configuring the radiation detector. The non-uniformity of the photoelectric charges gives rise to the non-uniformities described above. In tomosynthesis imaging, in which the irradiation duration of one shot of radiation is comparatively short (for example approximately 100 msec), such non-uniformities are not liable to become an issue, however in two-dimensional (2D) imaging, in which the irradiation duration of radiation is longer than that of tomosynthesis imaging (for example approximately 1 to 6 seconds), such non-uniformities become pronounced.